1. Field of the Invention
This invention relates to a process for at least partially separating a gaseous component from a mixture of gaseous components by alternating pressure adsorption and desorption on molecular sieves. This process essentially comprises an adsorption cycle and a desorption cycle, the adsorption cycle comprising a pressure increase step and an adsorption step, and the desorption cycle comprising a desorption step. When carbon molecular sieves are used as the adsorbent for the separation of nitrogen and oxygen, the oxygen is essentially adsorbed by the carbon molecular sieves during the adsorption cycle, while the nitrogen remains substantially unadsorbed. The nitrogen can then preferably be removed from the gas mixture under pressure as a product gas. During the desorption cycle, the oxygen, possibly after the performance of a pressure equalization step in a two-adsorber operation, is substantially desorbed by decompression from the carbon molecular sieves and can be released back into the atmosphere.
2. Background Information
Many such alternating pressure adsorption processes for obtaining nitrogen are known. Such processes are used in alternating pressure adsorption facilities which contain at least one adsorber, which adsorber is filled with the molecular sieves, and in which the adsorption and desorption cycles run in alternation.
There are essentially two basic types of alternating pressure facilities for the decomposition of air into nitrogen and oxygen. The first type utilizes zeolite molecular sieves, and the second type utilizes carbon molecular sieves (CMS).
In the alternating pressure facilities which operate with zeolite molecular sieves, the nitrogen is adsorbed on the inner surface of the pore system because of rather strong equilibrium adsorption forces, and the oxygen is obtained in an enriched form during the adsorption step.
On the other hand, the alternating pressure facilities which operate with carbon molecular sieves (CMS), have the characteristic that, in contrast to zeolite molecular sieves, the oxygen is essentially adsorbed inside the pores before the nitrogen can be adsorbed inside the pores, so that nitrogen can thus be obtained in a substantially enriched or highly-concentrated form as a product gas during the adsorption stage. The separation effect here, in contrast to the zeolites, is not based on the establishment of an equilibrium, but on the different diffusion velocities in the narrow pores of the carbon molecular sieves. The oxygen molecule at approximately 2.8 AE (or .ANG.), is somewhat smaller than the nitrogen molecule at approximately 3.0 AE (or .ANG.), and thus, the oxygen molecules can penetrate faster into the narrow pores of the carbon molecular sieves, which is essentially not possible for the larger nitrogen molecule, or at least it may only become possible if very long times are allowed for the adsorption to occur.
Diffusion velocities have been measured for some carbon molecular sieves and according to these diffusion velocities a nitrogen equilibrium can only be established after a period of about several hours or possibly even several days.
In accordance with the different separation mechanisms described above, different process conditions are preferably also employed when carbon molecular sieves are used. For example, in the process which utilizes carbon molecular sieves, nitrogen is essentially obtained in the adsorption step and not in the desorption step as essentially occurs in the process which utilizes zeolites. And since nitrogen essentially will, over time, also be adsorbed into the carbon molecular sieves, relatively short adsorption steps must also preferably be used, e.g., from about one to about ten minutes, so that a great deal of oxygen is adsorbed, while adsorbing as little nitrogen as possible. The adsorption generally takes place at pressures above about 1 bar, preferably between about 5 bar and about 12 bar, while a decompression into the normal pressure range (about 1 bar) is preferably generally performed for the desorption, and in certain applications possibly even into the vacuum range.
To optimize the process, an attempt is generally made to achieve the highest possible amount of nitrogen per cubic meter of CMS volume and per hour (m.sup.3 N.sub.2 /m.sup.3 CMS.h), simultaneously with the lowest possible air consumption per cubic meter and per hour (m.sup.3 L/m.sup.3 CMS.multidot.h). For the design of a pressure alternation process based on carbon molecular sieves, therefore, not only is the amount of nitrogen produced important, but also, the ratio of air used to nitrogen produced in important because the highest possible amount of nitrogen does not necessarily simultaneously mean a low air consumption. Overall, for the design of an alternating pressure facility, it may be more economical to use a carbon molecular sieve which produces somewhat lower quantities of nitrogen--specifically in relation to the CMS volume--which means a larger adsorber volume is essentially necessary for the production of the nitrogen in this facility--and to keep the specific air consumption, which is the determining factor for the size and power of the air compressor and thus the energy costs, as low as possible. In an individual case, therefore, to minimize the cost of generating nitrogen, the capital investment costs and the costs of operation (essentially electricity costs) must be taken into consideration so as to obtain the most favorable nitrogen generation price.
To optimize the alternating pressure processes on carbon molecular sieves, emphasis is generally placed on improving the characteristics of the molecular sieves, but emphasis can also be placed on improving the process technology. Generally, a vacuum desorption process, with its rather expensive investment in a vacuum generator, is no longer utilized, and instead the adsorption is conducted at somewhat higher pressures, e.g., at about 5 bar to about 12 bar, and desorption is performed by decompression to about 1 bar. To increase the nitrogen yield and to reduce the specific air consumption, multiple-adsorber facilities preferably take advantage of the capabilities of the pressure equalization between just-adsorbed and just-desorbed adsorbers, and also preferably take advantage of the recycling of a portion of the nitrogen produced before the next adsorption step. Since the raw gas, namely air, is essentially always going to be available in the environment in sufficient quantity, the conventional practice is to return the waste gas, namely the gas having a high oxygen content, to the atmosphere during the desorption step.
With modern alternating pressure processes to obtain nitrogen from air, the quantities of nitrogen and air requirements indicated in the following table, along with the ratios of air to nitrogen determined from them, can be achieved, whereby the residual oxygen concentration in the nitrogen product gas is shown to be about 0.5% and about 1.0% oxygen.
TABLE 1 ______________________________________ Source Specific N.sub.2 purity Air con- air con- (% O.sub.2 N.sub.2 quantity sumption sumption Process remaining) N L L/N ______________________________________ EP 00 85 155 0.5% 54 m.sup.3 /h 215 m.sup.3 /h 3.9 8 bar/1 bar 2 .times. 340 kg CMS at 680 kg/m.sup.3 DPS 27 02 784 1.0% 136 1/h = 490 1/h 3.6 8 bar/1 bar 66 m.sup.3 /h 239 m.sup.3 /h 2 .times. 700 g CMS at 680 g/l Advertising 1.0% 80 m.sup.3 /h 390.1 m.sup.3 /h 4.8 brochure* 6 bar/1 bar ______________________________________ *concerning a CMS product manufactured by the Kuraray Company, Japan
This invention relates to a process of the type described above for the separation of gaseous components from a mixture of gaseous components.